For the black-tailed godwit, true love is a long distance affair that hinges on an extraordinary bit of timing.

Pairs of this northern European species meet up in exactly the same place each year having spent many months and many kilometres apart. And the synchrony of the reunion has astonished researchers.

"The intriguing thing about these birds is that when they arrive back in Iceland to begin breeding each year, marked pairs of birds - male and female that have previously paired - get back to their territory almost at exactly the same time," Jennifer Gill, of the University of East Anglia, UK, told BBC World Service's Science In Action programme.

"There's only, on average, three days between the male arriving and the female - even though the whole population of godwits takes about a month to get back to Iceland."

Godwits (Limosa limosa islandica), which are wading birds, usually mate for life and can live for up to 25 years.

Ringed revelation

Males and females live in separate areas of northern Europe, in places as far apart as Britain, Portugal, Ireland and France.

In the spring, they fly up to Iceland, where almost the entire population breeds in the lowland areas.

The female does not wait around for very long to see if her male to show up - she'll move on and find somebody else

Jennifer Gill, UEA

Dr Gill said people had discovered other bird species that spend winters in several different parts of the globe but meet to breed in the same place.

But what makes the godwits exceptional is that the same pair finds each other year after year - and on time.

"If you think about how on Earth these birds manage to achieve that synchrony, the simplest solution would be for the birds to stay together throughout the year - to leave the breeding ground, travel south together, stay together through the winter and come back together," she said.

"What we found with godwits, because we have these marked rings on them and can follow them throughout the year, that's not actually the case - male and female godwits winter in completely different areas."

Route specialists

Knowledge of exactly when the pair are meant to meet is clearly essential to the relationship lasting - as shown by the occasions godwit males are observed to turn up late by mistake.

"In the two cases where we've recorded that so far, both have ended up in divorce," Dr Gill said.

A late godwit will be "divorced"

"So the female does not wait around for very long to see if her male will show up. She'll move on and find somebody else."

However, the researchers still do not know exactly how the birds achieve such precise timing and navigation.

"Migrant birds are known to be phenomenally good at selecting a route back to exactly the same location that they've been in previous years, and they use all sorts of cues to do that," she explained.

"But how they sort out the timing to such a degree of accuracy, we're really not sure."

Best conditions

It is suspected that the birds' abilities are related to the quality of wintering grounds - a good wintering site may encourage them to leave early.

"They could winter in areas of similar quality so they are ready to breed at the same time; there could be a genetic or psychological similarity between the paired birds; or they may time their arrival to coincide with the best conditions, exploiting peaks in food availability, for example," speculated Dr Gill.

"We suspect that this situation may be common to many other species of migratory bird as well - for which we unfortunately don't have the data."

The godwits' remarkable synchrony was recently reported in the journal Nature. The lead author on the paper was Tomas Gunnarsson.